The copending Application Ser. No. 870,397 filed Jan. 18, 1978, in the names of Day Chahroudi and John Brookes entitled "Transparent Infrared Radiation Reflection System" and assigned to the present assignee describes an energy-efficient material that reflects infrared (thermal) radiation incident thereon, yet is transmissive to visible and other solar radiation. Generally, the material includes a transparent flexible plastic substrate upon which is deposited a thin film of metal between layers of dielectric or semiconductor material. Contemplated as a substrate are such flexible plastics or polymer films as thin acrylics or those sold under the trademarks Mylar and Tedlar. It is contemplated that such substrate will have a thickness in the range of 0.5 to 20 mils while the thin film deposits will have a thickness between 50 and 2,000 angstroms.
There are at least three separate techniques presently known for depositing thin films of material of the thickness contemplated: The substrate may be coated chemically, by vacuum evaporation or by sputtering. Unfortunately, the chemical or vacuum evaporation deposition techniques do not lend themselves to coating large areas with film deposits of uniform optical thicknesses.
The sputtering technique has been found most useful of the three in coating large, long, flexible sheets of substrate with thin film deposits of uniform. Briefly, sputtering is a known metal deposition process that requires a cathode, an anode, a gas atmosphere for establishing a gas-discharge plasma (somewhat like that of fluorescent lighting), a source of sputtered material--the target (which is the cathode in this case) and an evacuable chamber to house these items. The chamber is partially evacuated to a pressure of anywhere from 1 to 100 millitorr and an electric field of several hundred volts established between the cathode and anode to produce glow discharge or plasma. The substrate is positioned within a few inches of the cathode (target), usually just outside the cathode dark space. Ions from the plasma are accelerated toward the cathode by the electric field where they strike and transfer their kinetic energy to atoms of metal, causing them to be ejected as a hot gas. This metal gas condenses on and coats the substrate.
If the sputtering process takes place in a gas that does not react with the atoms of the target (e.g., an inert gas such as argon) the coating deposited on the substrate will remain relatively pure. However, an admixture of a normally reactive gas (e.g., oxygen or fluorine) to the argon gas can be used to change the composition of the sputtered deposit to obtain, for example, an oxide or fluoride of the deposited metal.
In order to sputter deposit two or more layers of a material onto a flexible substrate of the type described above, each separate coating has been applied in a separate chamber in order to avoid having effluents from one sputtering process from contaminating the other. It was found that even with special care and handling techniques, which added to the cost of manufacturing the coated plastic, transferring the substrate from a chamber after one coating process to another chamber for another coating process tended to introduce impurity into the thin film deposits. It was soon realized that the time between deposits of the material onto the substrate was preferably minimized to thereby keep impurities without the particular thin film layer, be it metal or insulator. That is, when more than one coating is deposited onto a substrate, in order to minimize contamination, a great deal of effort generally goes into the isolation of each coating station from the other. Further, the acts of actually handling the substrate during the coating process (e.g., during transfer from one chamber to another) tends to produce a degree of surface roughening. This roughening or abrasion causes deleterious effects in the properties of the film deposited.
Accordingly, the present invention provides apparatus for sputter depositing two or more layers of material onto a flexible substrate, such as metal foil or plastic films, in a single chamber, yet isolating each individual deposition process from the other. Transfer of the substrate from one deposition chamber to another is obviated.
According to the present invention, therefore, there is provided an evacuable chamber containing three separate, individual deposition or sputtering stations arranged proximate a support member across which is moved the substrate by a transport mechanism. Each individual deposition station includes apparatus for sputtering a metal onto the substrate, and a housing that encloses the sputtering apparatus for containing the particular plasma used in the sputtering process. Each deposition station is provided with its own individual source of gas used to produce the plasma.
In the embodiment described herein, an evacuable chamber contains a support member, in the form of a cylinder that is rotatable about its longitudinal axis, and three individual deposition stations positioned at predetermined locations about the circumferential surface of support. The housing which encloses the sputtering mechanism of each deposition station opens toward the support member. The peripheral portions of each deposition station housing that define the opening are spaced a predetermined distance from the support member, forming an interstitial gap therebetween. This interstitial gap is dimensioned to allow the substrate to pass therethrough yet restrict effluent flow from the housing of the particular gas communicated to each individual deposition station. Further, the interstitial gap between the deposition station housing and substrate support is dimensioned so that a low probability of conductance of gaseous atom transmission from the chamber into the housing is obtained; that is, the smaller the gap, the lower the probability that gaseous atoms will find their way to and through the gap, into the housing interior. In addition, the gas introduced into the housing of each deposition station is maintained at a pressure that is higher than that of the chamber which, in turn, limits the level of gaseous contamination that can emanate from the chamber to a low value.
A transport mechanism, including a play-out roll and take-up roll, is also contained within the chamber to linearly advance the substrate along the support and past the series of three deposition stations.
The present invention effectively provides apparatus that almost simultaneously coats long, flexible sheets with multiple sputter depositions of material, yet minimizes the possibility of impurities produced by one deposition process effecting an adjoining deposition process. Thus, one deposition process involving a reactive metal being sputtered in a reactive atmosphere can take place simultaneously with and next to a second deposition process involving a metal being sputtered in an inert atmosphere.